This invention relates to code-division-multiple-access communications, and more particularly to power settings for a remote station, when initiating communications with a base station.
The terrestrial communications channel is typically non-reciprocal. If a base station 12 transmitted, as shown in FIG. 1, at a first power level P1 and at a first frequency f1, a first signal to a remote station 11, then the received power at the remote station 11 might be P11. If the remote station 11 transmitted, at the first power level P1 and at a second frequency f2, where the second frequency f2 is displaced from the first frequency f1 by more than a correlation bandwidth, then the received power at the base station 12 might be P12, which is statistically independent of the received power P11 at the remote station 11. The statistical independence, or non-reciprocal nature of the terrestrial communications channels, is of major concern to users of a code-division-multiple-access (CDMA) system.
In a direct-sequence (DS) CDMA system, a remote station""s spread-spectrum signal, received at a base station, is embedded in the interference caused by other users. Power control of the remote stations is therefore necessary for ensuring that during communications at the base station, the power level received from each remote station is approximately the same as from other remote stations communicating with the base station. Many elaborate systems exist for power control in a DS-CDMA system, where the base station determines the power levels of a received signal and interference, processes this information and periodically communicates to a remote station to increase or decrease its power level.
When a remote station is about to initiate its transmission, the remote station has little information as to what power level to transmit. Some investigators have suggested to use open-loop power control, in which the remote station monitors the power received from the base station transmitter at the first frequency f1, and from the monitored power, the remote station sets its initial power level of its transmitter. The remote station, however, transmits at a second frequency f2 which is not within the correlation bandwidth of the first frequency f1. Since the communications channel at the first frequency f1 is statistically independent from the communications channel at the second frequency f2, the open-loop power control does not work, or does not work well.
Another approach to power control is to initiate transmitting a remote station at a low power level, and periodically increase the power level of the remote station until a signal is received at the base station. When the power level from the remote station is sufficient for the base station to receive, then the base station sends a response to the remote station to stop increasing the power level, unless otherwise signaled to do so. While this approach works, it takes considerable time delay, particularly if packet transmissions are employed. Thus, a ten millisecond packet might last five seconds.
A general object of the invention is to permit a remote station to have knowledge, a priori to transmitting, of a proper power level to initiate transmission.
Another object of the invention is to measure and initially correct or compensate for Doppler shift in carrier frequency caused by the motion of the remote station.
According to the present invention, as embodied and broadly described herein, an improvement is provided to a spread-spectrum system which has a base station (BS) and a plurality of remote stations (RS). The base station has a BS-spread-spectrum transmitter and a BS-spread-spectrum receiver. The BS-spread-spectrum transmitter transmits, using radio waves, a plurality of BS-spread-spectrum signals at a first frequency. The BS-spread-spectrum receiver receives, at a second frequency, a plurality of RS-spread-spectrum signals from the plurality of remote stations. The plurality of BS-spread-spectrum signals at the first frequency are outside the correlation bandwidth of the plurality of RS-spread-spectrum signals at the second frequency. Each of the plurality of remote stations has an RS-spread-spectrum transmitter for transmitting an RS-spread-spectrum signal at the second frequency.
The improvement includes a BS transmitter and an interference-reduction subsystem, located at the base station receiver. The BS transmitter transmits, using radio waves, a BS-channel-sounding signal at the second frequency. The BS-channel-sounding signal has a bandwidth no more than twenty percent of the spread-spectrum bandwidth of the plurality of RS-spread-spectrum signals, and in a preferred embodiment, the BS-channel-sounding signal has a bandwidth no more than one percent of the spread-spectrum bandwidth of the plurality of RS-spread-spectrum signals.
At each remote station, the improvement includes an RS-power-level circuit and an RS receiver which has an RS demodulator. The improvement at the remote station also may include a frequency-adjust circuit. The RS receiver receives the BS-channel-sounding signal at the second frequency. The RS demodulator tracks the BS-channel-sounding signal, and outputs an RS-receiver signal. Using the receiver power level of the RS-receiver signal, the RS-power-level circuit adjusts an RS-power level of the RS-spread-spectrum transmitter located at the remote station. If the frequency-adjust circuit were employed, then the frequency-adjust circuit, using the received RS-receiver signal as a reference, compensates to the first frequency the RS-spread-spectrum signal of the RS-spread-spectrum transmitter located at the remote station.
The interference-reduction subsystem is located at the base station and at a front end to the BS-spread-spectrum receiver. The interference-reduction subsystem reduces, at the second frequency, the BS-channel-sounding signal from the plurality of RS-spread-spectrum signals arriving at the base station.
Additional objects and advantages of the invention are set forth in part in the description which follows, and in part are obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention also may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.